This invention relates to triisocyanates which contain both aromatically and cycloaliphatically bound isocyanate groups and to a process for their preparation.
Araliphatic polyisocyanates such as the isocyanate group-containing mixed primer of hexamethylene diisocyanate and diisocyanatotoluene are eminently suitable for the production of polyurethanes (particularly polyurethane lacquer coats) because the positive properties of aromatic isocyanates combine advantageously with those of aliphatic isocyanates. Polyurethane lacquers based on such araliphatic lacquer polyisocyanates are distinguished by the following advantageous properties (see H. Wagner, H. F. Sarx, "Lackkunstharze", publishers Carl Hanser Verlag, Munich, 5th Edition, 1971, pages 163-164): (1) rapid drying; (2) increased resistance of the lacquer coats to yellowing compared with those based on aromatic polyisocyanates; (3) increased reactivity and hence shorter drying times compared with corresponding lacquers based on aliphatic polyisocyanates; (4) good stability of the color shade of pigmented lacquers; (5) improved gloss retention and weathering resistance compared with corresponding lacquers based on aromatic polyisocyanates and (6) relatively long pot life of the lacquers.
The preparation of such araliphatic polyisocyanates by methods such as that described in German Patentschrift No. 1,670,667 is, however, rather involved, because the excess starting diisocyanates present after trimerization must be removed in an additional process step. Another disadvantage of such methods is that the trimerization reaction uses up valuable isocyanate groups. Further, the mixed trimers formed by these processes cannot be purified by distillation after their preparation.